The invention relates to a method for monitoring the speed of a synchronous motor, the synchronous motor having a permanent magnet rotor and coils, sinusoidal currents being applied to the coils, and to a device for monitoring the speed of such a motor.
In synchronous motors, the coils to which current is applied generate rotating magnetic fields which set the permanent magnet rotor rotating, the speeds of the magnetic field and rotor being the same. Known for the purpose of monitoring the speed of the rotor are sensors which detect the position of the rotor magnetically or optically. The sensors require additional installation space and necessitate overall a higher outlay on producing the motor.
It is therefore the object of the invention to specify a method for monitoring the speed in which it is possible to dispense with additional installation space and a higher outlay on producing the motor. An appropriate device is also to be specified.
These objects are achieved, on the one hand, by a method in which in the case of at least one of the coils the power supply is disconnected and a voltage characteristic is evaluated which is induced by the rotor in the coil or in one or more of the coils and not supplied with power, the minimum duration of the disconnection of the power supply and the evaluation of the voltage characteristic being equal to the temporal spacing between two consecutive zero crossings of the voltage respectively induced by the rotor.
In a device according to the invention for monitoring the speed of a previously described synchronous motor, the power supply of at least one coil can be disconnected, and an evaluation circuit is present in which it is possible to determine the speed of the rotor by evaluating the voltage characteristics which are induced by the permanent magnet rotor in the coils disconnected from the power supply.
The evaluation method is particularly simple in form when the coils disconnected from the power supply are isolated from the power supply.
By virtue of the fact that in each case two consecutive measurements of the voltage characteristic in a coil, or time-offset measurements of the voltage characteristics in a plurality of coils are carried out during a power supply interruption, it is possible to determine the deceleration of the permanent magnet rotor which is not, or partially not being driven.
The speed and the position of the permanent magnet rotor can then be determined from this deceleration for a subsequent instant, with the result that the coils can once again be driven optimally at the correct frequency and with the correct phase angle of the current.
The device according to the invention can be provided in a particularly simple fashion when at least one changeover switch is present with the aid of which one or more coils can be isolated from the power supply unit and can be connected to the evaluation circuit.